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United States Patent |
6,031,970
|
Nordal
,   et al.
|
February 29, 2000
|
Infared emitter and methods for fabricating the same
Abstract
An infrared radiation source for sensor and spectroscopic use has a thin,
electrically conducting film adapted to emit infrared radiation when
heated. The film is formed from a gas or vapor phase and includes a
network of diamond-like carbon. Depending on the percentage of metal atoms
in the film, the film may have metal atoms which are either distributed in
the diamond-like carbon network or else form an additional metallic
network. The metal may be tungsten, chromium, or titanium, and the film
may include silicon and oxygen.
Inventors:
|
Nordal; Per-Erik (Nesbru, NO);
Skotheim; Terje (Tucson, AZ)
|
Assignee:
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Patinor A/S (Oslo, NO)
|
Appl. No.:
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029836 |
Filed:
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July 1, 1998 |
PCT Filed:
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September 6, 1996
|
PCT NO:
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PCT/NO96/00216
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371 Date:
|
July 1, 1998
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102(e) Date:
|
July 1, 1998
|
PCT PUB.NO.:
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WO97/09593 |
PCT PUB. Date:
|
March 13, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
392/407; 219/553; 250/504R; 977/DIG.1 |
Intern'l Class: |
H05B 003/00 |
Field of Search: |
392/407
219/553,543
250/504 R
428/408
|
References Cited
U.S. Patent Documents
3961155 | Jun., 1976 | Weldon et al. | 219/543.
|
5128514 | Jul., 1992 | Lehmann et al.
| |
5352493 | Oct., 1994 | Dorfman et al.
| |
5488350 | Jan., 1996 | Aslam et al. | 219/553.
|
5705272 | Jan., 1998 | Taniguchi | 219/553.
|
Foreign Patent Documents |
503 934 | Sep., 1992 | EP.
| |
533 211 | Mar., 1993 | EP.
| |
588 449 | Mar., 1994 | EP.
| |
632 344 | Jan., 1995 | EP.
| |
64-56401 | Mar., 1989 | JP.
| |
6-348154 | Dec., 1994 | JP.
| |
149679 | Feb., 1984 | NO.
| |
WO 90/14580 | Nov., 1990 | WO.
| |
Primary Examiner: Jeffery; John A.
Attorney, Agent or Firm: Browdy and Neimark
Claims
We claim:
1. Infrared radiation source for sensor and spectroscopic applications
comprising a thin, electrically conducting film adapted to emit infrared
radiation on heating, wherein the film is formed from gas or vapour phase
deposition, characterized in that the film comprises at least one network,
the at least one network being a network of diamond-like carbon.
2. Infrared radiation source according to claim 1, characterized in that
the film comprises metal atoms distributed as separate atoms in the
diamond-like carbon network.
3. Infrared radiation source according to claim 2, characteized in that the
metal atoms are selected among the elements tungsten, chromium and
titanium.
4. Infrared radiation source according to claim 1, characterized in that
the film comprises an additional network of silicon and oxygen.
5. Infrared radiation source according to claim 1, characterized in that
the film is provided on a supporting substrate.
6. Infrared radiation source according to claim 5, characterized in that
the supporting substrate is an aerogel.
7. Infrared radiation source according to claim 1, characterized in that
the film is self-supporting.
8. Infrared radiation source according to claim 1, characterized in that
the film has an electric resistance of between 50 and 400 ohm per square.
9. Infrared radiation source according to claim 1, characterized in that
the surface of the film has been provided with an emission layer from gas
or vapour phase deposition.
10. Infrared radiation source according to claim 1, characterized in that
the film comprises two or more areas, each being connected with
respective, separate electric driving electrodes.
11. Infrared radiation source comprising a thin, electrically conducting
film according to claim 1 and adapted to emit infrared radiation upon
being heated, wherein the film is formed from gas or vapour phase,
characterized in that the surface of the film comprises topographic
microstructures.
12. Infrared radiation source according to claim 11, characterized in that
the microstructures are in the form of parallel grooves in the surface of
the film.
13. Infrared radiation source according to claim 11, characterized in that
the microstructures are in the form of a grid with two or more step levels
or tiers.
14. Infrared radiation source according to claim 1, characterized in that
the film comprises metal atoms forming an additional metallic network.
15. A method for fabricating an infrared radiation source according to
claim 1, wherein the radiation source comprises a film the surface of
which comprises topographic microstructures, characterized in depositing
the film on a substrate, the surface of the substrate comprising
topographic microstructures corresponding to those which are to be formed
on the surface of the film, whereby the microstructures on the surface of
the film are formed during the deposition of the film on the substrate.
16. A method for fabricating an infrared radiation source according to
claim 15, characterized in that the topographic microstructures in the
surface of the substrate are formed by stamping or moulding.
17. A method for fabricating an infrared radiation source according to
claim 15, characterized in that the substrate is formed by a polymer.
18. A method for fabricating an infrared radiation source according to
claim 15, characterized in that the film is deposited on a substrate which
is removed after the deposition of the film.
19. A method for fabricating an infrared radiation source according to
claim 18, characterized in that the substrate is formed by fluourcarbon
which is removed by exposure to UV-radiation.
20. A method for fabrication of an infrared radiation source according to
claim 18, characterized in that the substrate is formed by a polymethyl
methacrylate (PMMA) polymer which is removed by heating.
21. A method for fabricating an infrared radiation source according to
claim 18, characterized in that the substrate is removed after the
deposition of the film by vaporization under influence of heat and
subsequent sublimation.
22. A method for fabricating an infrared radiation source according to
claim 18, characterized in that the substrate is removed after the
deposition of the film by vaporization under influence of UV radiation and
subsequent sublimation.
23. A method for fabricating an infrared radiation source according to
claim 18, characterized in that the substrate is removed after the
deposition of the film by being dissolved by exposure to chemical attack.
24. A method for fabricating an infrared radiation source according to
claim 18, characterized in that the substrate is removed after the
deposition of the film by being gassified by exposure to chemical attack.
25. A method for fabrication of an infrared radiation source according to
claim 15, characterized in that electric feed conductors and a mechanical
suspension means for the film are embedded in the substrate before the
film is deposited.
Description
FIELD OF THE INVENTION
This invention describes a class of infrared radiation emitters suitable
for applications in spectroscopic devices and instruments as well as in
thermal printers, etc.
BACKGROUND OF THE INVENTION
There are known emitters where the infrared radiation emanates from a
surface which is directly or indirectly heated by an electrical current
passing through a conducting layer in or below the surface. The elevated
temperature leads to increased emission of radiation, with the intensity
and spatial as well as spectral distribution of the emitted radiation
depending on the element temperature, as well as on its emissivity and
surface topography.
For a greybody with emissivity .epsilon., the emitted power per unit area,
within a wavelength interval d.lambda. at wavelength .lambda. is given by:
##EQU1##
where T is the temperature, h is Planck's constant, k is Boltzmann's
constant and c is the velocity of light.
In spectroscopic analysis, parts of the emitted power within restricted
wavelength regions are selected by means of optical filters, and the
radiation source is sought optimized by having a high emissivity .epsilon.
and high temperature T.
In order to achieve compact, efficient and low-cost device solutions, it is
desirable to switch or modulate the emitted radiation by rapidly varying
the temperature T of the emitter, rather than by mechanical motion of
shutters, filters etc. Electrically switched infrared emitters are also of
great interest in many non-spectroscopic applications such as thermal
printers, etc.
Two well-known types of pulsed thermal emitters in current use are:
1) Filaments in bulb-like enclosures. The filament is driven by a pulsating
current, causing heating/cooling and associated variations in emitted
infrared radiation. These sources are used in infrared sensors for gas
monitoring, etc, and are relatively cheap. Unfortunately, their operative
life is short, requiring frequent replacement. Also, they are bulky and
consume much electrical power compared to the useful infrared radiation
emitted.
2) Electrically conducting, planar surfaces that are heated by a pulsating
electrical current. The surface may be a flat, insulating substrate which
is coated by a conducting film or layer, or it may be a thin, freely
suspended membrane of a material which is itself electrically conducting.
The former type of emitter is described in Norwegian patent No. 149,679 and
U.S. Pat. No. 3,961,155. Drawbacks of these types of emitters include:
Poor efficiency (i.e. much electrical power needed compared to the useful
radiation emitted), which is in large part due to heat conduction through
the relatively thick (typically 0.5 mm) substrate. Also, mechanical
mounting and bonding of electrical connections is critical and labor
intensive.
The latter type has been implemented in commercial gas sensors, in the form
of a silicone membrane which is etched thin and doped to high conductivity
in a central region where infrared emission takes place. These sources
have proven very robust and long-lived, and are generally more efficient
than the emitters referred above, although the heat loss through the
membrane to the mounting fixtures is still quite high. This is due to the
thicker portions of the membrane, which must have a certain mechanical
strength to permit an acceptable yield during manufacturing operations, as
well as robustness in practical use. A serious drawback of these emitters
is their high manufacturing cost.
The ideal thermal emitter should convert 100% of the dissipated electrical
energy into infrared radiation at the desired wavelengths. Membrane
emitters described above are a far cry from this; typically the ratio
between total radiated power and supplied electrical power is 10% or less.
The pulsating part of the radiated power is a fraction of the total
emitted power, and the infrared emission within specific spectral bands is
a fraction of this.
As shown schematically in FIG. 1, heat is generated in the thin membrane
and can take several different paths:
a) Radiation into the space in front and back of the membrane.
Radiation is the heat loss mechanism which is sought maximized at the
expense of the following:
b) Conduction through membrane mounts and electrical connectors.
c) Thermal diffusion through gas surrounding the membrane.
d) Convection in gas surrounding membrane.
Each of these can be quantified, subject to defined conditions such as
materials, dimensions and operating temperature. Consider, e.g. the
membrane shown in FIG. 2, which is freely suspended at opposite ends and
surrounded by parallel surfaces representing the floor and the output
window in a can filled with air: The different heat loss contributions a)
to d) are shown qualitatively in the graph.
As is evident from FIG. 2, and by simple intuitive reasoning, the membrane
emitter should have certain basic features:
First, in order to emit much infrared radiation, the membrane temperature
and emissivity should both be high, cf. Equation 1). More specifically,
the temperature should reach a high peak and a low trough value during
each temperature cycle (cf. FIG. 3), to maximize the difference in emitted
radiation between the <<power on>> and <<power off>> states. High
temperature contrast is obtained by:
Inducing a rapid temperature rise during the application of electrical
power to the membrane. This in turn is facilitated by high power
dissipation and low heat capacity in the membrane.
Achieving rapid cooling of the membrane during the <<power off>> period.
This is achieved by a low heat capacity in the membrane, coupled with high
heat loss from the membrane. A high heat loss from the membrane by
mechanisms other than a) above, leads too poor efficiency, however.
Summing up, the heat capacity of the membrane should be as low as possible,
implying in practice that the membrane should be as thin as is compatible
with mechanical strength requirements in the given application (vibrations
etc). Furthermore, the surface facing the direction in which the radiation
is to be emitted should have a high emissivity at the wavelengths of
interest. Total heat transport from the membrane should be minimized, but
must be sufficiently high to yield a sufficiently rapid cooling transient
during the <<power off>> period. In the limit of an extremely thin
membrane with near-zero heat capacity, heat loss by radiation is
sufficient to cause a rapid cooling transient, and all other heat loss
mechanisms b) to d) should be brought as close to zero as possible.
One notes that a thin membrane is all-critical, since this reduces the need
for heat loss by other mechanisms than a), and leads to an efficient
device requiring little electrical power. Heat transport along the
membrane and to mounting posts/electrical leads is also minimized by using
a thin membrane.
Membrane sources with their shortcomings as outlined above represent a
compromise between the ideal technical requirements outlined above and the
need for physical robustness and manufacturability at acceptable costs.
SUMMARY OF THE INVENTION
According to the present invention there is provided an infrared radiation
emitter the claims. A method for manufacturing the i.r. emitter is
disclosed by the characterizing part of claim 14. The invention is
described in greater detail below, with references given to the
accompanying drawings:
BRIEF DESCRIPTION OF THE DRAWINGS FIGURES
FIG. 1: Heat transport paths away from the heated region:
a) Infrared radiation.
b) Conduction through membrane mounts and electrical connectors.
c) Thermal diffusion through gas surrounding the membrane.
d) Convection in gas surrounding the membrane.
FIG. 2: Principle sketch of self-supporting membrane emitter in typical
temperature dependence of dominant heat transport mechanisms from
membrane.
FIG. 3: Time evolution of applied electrical power and resulting membrane
temperatures.
FIG. 4: Stages in the manufacturing of radiation source with self
supporting membrane:
a) Membrane substrate ready for deposition of DLC.
b) DLC deposition.
c) Substrate removed.
d) Finished source package.
FIG. 5: Example of surface that is structured to achieve high emissivity.
FIG. 6 a,b,c:
Examples of surfaces that are structured to control spectral (a,b) and
polarization (c) properties of emitted radiation.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on a thin emitter layer consisting of
conducting diamond-like carbon (DLC). Diamond-like carbon materials and
some applications are e.g. described in U.S. Pat. No. 5 352 493 (Dorfman
et al.) which is hereby incorporated herein in its entirety by reference.
Thin, self-supporting membranes are prepared from gas phase by deposition
of DLC on a substrate which is subsequently eroded away, leaving the DLC
membrane electrically connected and supported by posts previously embedded
in the substrate which was removed. This procedure, combined with the
extreme mechanical strength of the DLC material, permits ready-mounted
membrane emitters to be manufactured with large area-to thickness ratios,
in a few and simple manufacturing steps.
Alternatively, a DLC emitter layer is formed on top of a substrate which
has low thermal diffusivity, and which remains in place after completion
of the DLC deposition process. As in the previous case, electrodes are
pre-embedded in the substrate and become electrically connected with the
membrane during DLC deposition.
By employing substrates with a microtextured surface, DLC membranes are
formed with surface topographies that confer special emission
characteristics upon the membrane, i.e.: high emissivity, spectral
selectivity and/or emission of polarized radiation.
During recent years, a range of technologies for producing DLC films have
been developed. Such Films can be made with physical and chemical
properties that are very useful in a vide range of applications. An
example of the latter is given in European Patent EP-A-0 588 449, where an
electric field effect light emitting device is described, based on an
oxygen-containing diamond-like film. The film further comprises metal
atoms in order to tune the device to the desired wavelength, e.g. Cr or Fe
for the red band. The device shows an excellent moisture resistance, long
service life and high luminance. In contrast to the present invention,
however, the device is based on short-wavelength (visible) radiation being
generated by electroluminescence, whereas the present invention invention
describes predominantly infrared emission that is generated thermally.
Of particular interest here are films that:
Are deposited at moderate to low temperatures, and thus can be coated on
polymers and other heat-sensitive materials without incurring deformation
or damage.
Are electrically conducting. In U.S. Pat. No. 5,352,493 (Dorfman et al.) is
described a class of of metal-containing DLC films, termed diamond-like
nanocomposites (DLN), that have a number of beneficial properties relative
to the present application. Diamond-like nanocomposites consists of
interpenetrating networks of predominantly sp3-bonded carbon stabilized by
hydrogen, glass-like silicon stabilized by oxygen, and random networks of
elements from the 1-7b and 8 groups of the periodic table. Thus, the
electrical conductivity can be adjusted continuously over 18 orders of
magnitude by varying the concentration of built-in metal atoms.
Conductivities as high as 10 000 S/m have been achieved, while preserving
the diamond-like nature of the material.
An important consideration is the ability of the membrane to withstand high
temperatures and thermal cycling. In an oxygen-containing atmosphere, pure
diamond oxidizes at temperatures above 700-750.degree. C., while typical
amorphous diamond-like materials deteriorate much more rapidly. The
metal-containing DLN films, however, have high-temperature stability
comparable to or better than that of crystalline diamond, and exhibit
extreme stability during thermal and mechanical cycling. In air, estimated
long term stability at 450.degree. C. is more than 10 years, and DLN
thermal resistors have withstood 2.times.10exp8 pulses with maximum
temperatures above 750.degree. C., and 10exp6 pulses with maximum
temperatures above 1250.degree. C. In an oxygen-free environment, films
are stable up to 1000.degree. C. for several hours and up to to
1500.degree. C. for short pulses.
Current commercial membrane emitters based on other technologies typically
operate at temperatures up to 650.degree. C., and are enclosed in a sealed
can containing an inert atmosphere.
The basic manufacturing steps are sketched in FIG. 4:
a) The base of the can in which the membrane emitter is to be housed is
temporarily prepared with a substrate as shown before being exposed to DLC
deposition. Here, electrical leads enter the can through its base and end
flush with the top of the substrate, where they are shaped so as to define
the supporting structure for and the electrode connections to the DLC
membrane.
b) The substrate is subjected to DLC deposition. All surfaces that are not
to remain coated by the DLC film are either shielded, or coated by a
protective layer which resists formation/adhesion of the DLC film. The
part of the film that is formed on top of the electrodes becomes
electrically connected in the deposition process.
c) The substrate and shielding materials are removed, leaving the membrane
suspended and electrically connected.
d) The can is closed and backfilled with inert gas, or evacuated and
sealed.
Step c) is critical, in particular the removal of the substrate. The DLC
layer is extremely thin and must not be subjected to undue mechanical
stress during this process. At the same time, it is desired that removal
of the substrate shall be as simple and reliable as possible, and there
should be no residue.
A solution according to the present invention is to employ substrates that
can be dissolved or gasified by chemical and/or physical processes that do
not attack other components in the emitter assembly. This is possible,
since there exist DLC deposition processes that do not subject the
substrate to high temperatures. This makes a wide field of materials
accessible for use as substrates.
As an example, it is known that PMMA depolymerizes at temperatures of
300-350.degree. C. and above, going into a monomer gas phase and leaving
no residue. This is below the anticipated operation temperature in most
applications, and well below the failure temperatures of DLC materials of
interest. PMMA is also attractive as a substrate material, since
substrates in this material can be made with a high degree of control,
including microtexturing (cf. below).
A number of alternatives exist, where gasification can be effected by
thermal and/or chemical means, e.g. ozone erosion of polymers.
The primary reason for removing the substrate to leave a free-standing
membrane as described above is to minimize heat loss by thermal diffusion.
Given a substrate with the right physical characteristics, however, it
might be preferable to let it remain as an integral part of the emitter
assembly: Manufacturing would not require the substrate removal step, and
the substrate would act as a mechanical support during vibrations, etc.
A substrate that can be left in situ following DLC deposition must fulfil
several requirements, including low differential thermal expansion
relative to the DLC film, good resistance to thermal degradation, and good
adhesion to the DLC film. Particularly important, however, is that it
should be a poor heat conductor, such that a high average temperature in
the DLC film does not lead to large heat loss through the substrate.
Furthermore, the heat capacity in the layer within one thermal diffusion
length .mu. from the DLC film should be minimized also: The temperature
excursions in the film make themselves felt down to approximately this
depth in the substrate. Thus, this layer increases the apparent heat
capacity of the DLC film, reducing the temperature contrast during
pulsing.
One material that fulfils the basic requirements defined above, is silica
aerogel. It is a silicate network with hitch pore content, characterized
by extremely low heat conductivity (typically down to 1.5 10exp(-4) [W/cm.
K]), specific heat and --weight (down to 0.003 [g(cm3]). Thermal expansion
and resistance to heat degradation are comparable to those of glass. The
material is low cost and lends itself well to shaping and mounting in an
emitter structure. The surface topography can be controlled over a wide
ranges minimum feature size being defined by a pore size in the range 1 to
100 nm. Desired surface structures can, e.g. be made by mechanical
processes such as machining, or by moulding during production of the
aerogel.
The silica aerogel has a connected pore network. i.e. it can be used with
an emitter film in a can which is either evacuated or filled with an inert
gas. In the latter case, the aerogel substrate shall prevent convective
heat loss by gas circulating past the back of the membrane, which is a
potential loss mechanism with self-supporting membranes in a gas-filled
can.
Since much of the heat conduction through aerogel is radiative, it is
common to add IR absorbing material to the matrix. In the present case,
this would enhance the emitter efficiency by re-radiation back into the
DLC film, and could contribute to the emissivity of the latter.
A basic problem with self-supported membrane emitters in the thickness
range from sub-micron to a few microns, is that even membrane materials
that are usually considered opaque start to transmit infrared radiation
when they become this thin, and their emissivity becomes low also. Even
when suitable materials and processes exist, coating the membrane with a
high-emissivity film adds complexity and cost, and may add too much to the
heat capacity.
This problem is well-known from the design of infrared bolometer and has
been studied in that connection (see, e.g. in Refs. 1,2,3) See, e.g., the
following references: J. Clarke et al., "Superconductive bolometers for
submillimeter wavelengths", J. Appl. Phys. 48, 4865 (1977); K. C.
Liddiard, "Thin-film resistance bolometer IR detectors", Infrared Phys.
24, 57 (1984); M. Dragovan and S. H. Mosely, "Gold absorbing film for a
composite bolometer", Appl. Opt. 23, 654 (1984). Another reference is D.
H. Raguin and G. M. Morris, "Antireflection structured surfaces for the
infrared spectral regions", Appl. Opt. 32, 1154 (1993). in Ref. 3, a
dielectrial (diamond flake) was gold coated to an optimum electrical
resistance per area to obtain high absorption. A broadband single pass
absorptivity up to 50% is achievable by metallization, and considerably
higher at selected wavelengths in resonant structures.
In commercial membrane emitters based on silicon, the electrical
resistivity is controlled by doping (typically in the range 100 to 200
ohms per square in membrane) to obtain the optimal emissivity.
In the case of electrically conducting DLN films, it is possible to adjust
the emissivity across a wide range by choosing metal atom concentration.
This is an intrinsic feature to the film, and no further coating steps
should be necessary.
The surface structure may profoundly affect the emissivity. Thus, a surface
structure as shown in FIG. 5 enhances the broadband emissivity by:
a) Cavity effect (compare: high absorption by multiple reflections of
incident radiation)
b) Antireflection effect (if structure details are smaller than the IR
wavelength emitted, it acts as an index match between substrate and the
surrounding air or vacuum, cf. Ref. 4).
As is immediately apparent from the description of the preparation
technique illustrated in FIG. 4, it may be advantageous to employ a
substrate with microtexture, e.g. as shown in FIG. 5, to enhance the
emissivity of the membrane emitter layer. This applies also in the case
where the substrate is not removed following DLC deposition. For a wide
range of substrate materials, e.g. polymers, the desired surface structure
can be created by simple stamping or moulding. The DLC film deposition
process and the thickness of the layers laid down shall generally be such
that the film follows the substrate contour with high fidelity.
As described so far, the topographic surface structures serve to enhance
the broadband emissivity of the surface. It is possible, however, to
control the film's physical parameters so as to obtain enhanced emissivity
at certain wavelengths. An example of this, applicable with emitter
membranes of relatively low absorptivity (with or without a conductive
coating on one side) is to choose the emitter thickness so as to take
advantage of Fabry Perot resonances within (cf. e.g. Ref. 1). Due to the
high degree of control during the deposition process DLC films should lend
themselves well to such strategies. With membrane materials of high
intrinsic absorption, however, the enhancement and spectral selectivity
shall be modest, and one may consider the following alternative:
For a material with very high emissivity and with a surface topography as
shown in FIG. 6a, one finds that the emitted radiation shall be enhanced
at wavelengths yielding constructive interference between emission from
top and bottom of the steps, provided that light is collected from a
portion of the surface containing a large number of steps. While this idea
is novel in the emission context, analogous effects are well known in
connection with echelon gratings, where high wavelength selectivity is
obtained.
In order to physically make emitters exploiting this principle according to
FIG. 6a, one can see that high aspect ratios must be achieved; i.e. deep,
but narrow steps. This is generally difficult, even in the case of DLC
deposition. An alternative structure that takes advantage of the
equivalence of waves phase shifted by a multiple of 2.pi., is shown in
FIG. 6b. In this case, one must take into account possible sidebands
created by the more coarse sub-structure. Instead of a binary structure,
one may employ a multiple step height structure, etc. Thus, surface
structuring provides a new dimension of freedom for tailoring the spectral
emission characteristics of the emitter surface. Both with regards to
design and manufacturing, one may expect synergy with diffractive optics
technology which is currently evolving rapidly.
Going a step further, one may consider the structure shown in FIG. 6c,
where the steps are elongated in one dimension, extending as parallel
features across macroscopic portions of the emitter surface. As can be
easily understood in cases where the top of each ridge is made highly
electrically conducting (e.g. by skew metallization), the radiation from
this surface shall have a high content of linear polarization. References:
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